1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a method for assembling a turbine frame assembly of a gas turbine engine.
2. Related Art
Conventional high bypass ratio turbofan engines, which are included in the more general category of gas turbine engines and which may be used for aircraft propulsion, typically include a fan, booster, high pressure compressor, combustor, high pressure turbine and low pressure turbine in serial axial flow relationship about a longitudinal centerline axis of the engine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, with the rotatable portions of the combination of components comprising a high pressure rotor module. The low pressure turbine is drivingly connected to both the fan and booster via a second rotor shaft, with the rotatable portions of the combination comprising the low pressure rotor module. The high and low pressure rotor modules am supported by bearings which, in turn, are supported by structural frame components such as the turbine center frame assembly which is positioned between the low and high pressure turbines.
Structural frame components typically include an annular inner hub, an annular outer casing and a plurality of circumferentially spaced and radially extending hollow struts which extend between and are fixedly connected to the inner hub and outer casing. The outer casing, struts and inner hub may be made of an integral casting or, in other known frame assemblies, the inner hub and radially extending struts may be made of an integral casting with the struts bolted to the outer casing.
Structural frame components which am disposed downstream of the core engine, such as the turbine center frame assembly, are exposed to the hot combustion gases of the primary or core engine gas stream which are produced when the pressurized air exiting the high pressure compressor enters the combustor and is mixed with fuel and burned to provide a high energy gas stream. The high energy gas stream then expands through the high pressure turbine prior to engaging the turbine center frame assembly. Since the struts of such frame assemblies must pass through the combustion gases they must be suitably protected from the high temperature gas stream. Accordingly, each of the struts are typically encased by air-cooled aerodynamically-shaped fairings.
Structural frame assemblies must have suitable structural rigidity for supporting the rotor shafts of the gas turbine engine so as to maintain rotor to stator concentricity and the associated clearances within acceptable limits. Known examples of structural frame assemblies used in earlier gas turbine engines are configured so that the radially extending struts penetrate the outer annular casing. However, this penetration is known to decrease the effective rigidity of the structural frame assemblies. Consequently, known examples of structural frame assemblies used in more modem gas turbine engines, such as that disclosed in U.S. Pat. No. 5,292,227, filed Dec. 10, 1992, entitled "TURBINE FRAME", which is assigned to the assignee of the present invention and which is herein expressly incorporated by reference. U.S. Pat. No. 5,292,227 discloses a turbine frame assembly 32 which includes a first structural ring 36 or outer annular casing, a second structural ring 38, or inner hub, and a plurality of circumferentially spaced apart hollow struts 40 extending radially between the outer casing and inner hub 36 and 38, respectively. In the exemplary embodiment illustrated in FIG. 1 of U.S. Pat. No. 5,292,227, the inner ends 40b of the struts 40 are integrally formed with the hub 38 in a common casting, for example, and the outer ends 40a of the struts 40 are removably fixedly joined to the casing 36 using clevises 52, with each clevis 52 including an arcuate base 54 disposed against the tuner circumference of the casing 36. Clevises 52 further include a plurality of mounting holes 56, with eight being shown for example, for receiving a respective plurality of mounting bolts 58, with corresponding nuts, therethrough to removably fixedly join the base 54 to the casing 36. U.S. Pat. No. 5,292,227 further discloses that for increased rigidity of the turbine frame assembly 32, and to insure repeatability of reassembly, the clevis 52 and strut end 40a may be ground to establish an interference fit to the casing 36. The interference fit and the inaccessibility of struts 40 from inside the casing due to fairings 42 which surround respective ones of the struts 40, create difficulty in aligning casing 36 relative to struts 40 which is required to install mounting bolts in line-drilled holes 56 extending through casing 36 and clevises 52. This alignment is exacerbated, as may be appreciated by one of ordinary skill in the art, due to the frusto-conical shape of casing 36.
In view of the foregoing, prior to the present invention, a need existed for a method of aligning an outer casing of a turbine frame assembly such as that disclosed in U.S. Pat. No. 5,292,227.